sql server performance got slow down suddenly.

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SQL

SQL-Server-2008R2

hello Friends...

i m using sql server 2008 on windows server 2008 r2. it contains a database named 'muri'. and i have created a linked server to proficy historian using iHistorian OLE DB Provider. i executed some query for linked server. It was working Fine till morning.
when i run simple following query.

select * from openquery(ihist,'select * from ihtags')

it went executing until i close the sql server.It takes long time but don't complete execution. first i thought sql server went slow down. but when i run simple query to my muri database as follows.

select * from employee

it is working perfect. after that i tried to delete existing linked server, this also showed progress as executing but don't complete execution and deletion.
then i tried to create new linked server with different name, same thing happened with this also.
it can't delete existing linked server,can't create new linked server and can't execute openquery as follows

select * from openquery(ihist,'select * from ihtags')

plz help.
thank you